I know where the save files are on my PC, (it's nearly 1Gb,btw). What i want to know, is there a way of changing the file file name to a custom name. I would like to make saves named based on how i'm playing , eg: NG+ death march mode, or, fresh new game, any mode.

It seems a bit tricky since the save file names are hexadecimal based. Anyone have suggestions?

As you noted, the game keeps track of saves with a hexadecimal convention. Renaming them is impossible. However, if you're wanting to keep track of different profiles you could try this mod.

Just rename the save files as you want. Game will read the files. Spaces and all, but ingame it will always be lower case.

If you have cloud sync on and want to sync the files to the cloud, alt-tab from the game, make the changes you want, alt-tab in and exit, the cloud should be synced as-is now.

Oh and rename the .png same as the .sav so the thumbnail will work

I tried this and the game never recognised the renamed saves.

Weird, I'll take a couple of screens tomorrow to prove myself. I'm using GOG goty and obviously the most recent version.

Here's how you do it:
https://imgur.com/gallery/sjOqfi1

Hmm, you're right. What I did originally was to copy the save game and .png and rename the copy, but when you do that the game doesn't recognise the copied save. If you rename the original file it's recognised. Interesting.